The present invention relates to a fuel injector.
German Published Patent Application No. 33 14 899 describes an electromagnetically actuatable fuel injector, in which an armature cooperates with an electrically excitable solenoid coil for the purpose of electromagnetic actuation, and the stroke of the armature is transmitted via a valve needle to a valve-closure member. The valve-closure member cooperates with a valve seat. The armature is not fixedly mounted on the valve needle but rather the armature is arranged so as to be axially movable with respect to the valve needle. A first resetting spring acts upon the valve needle in the closing direction and therefore keeps the fuel injector closed in the currentless, non-excited state of the solenoid coil. The armature, via a second resetting spring, is acted upon in the stroke direction such that the armature, in the resting position, contacts a first limit stop that is provided on the valve needle. When the solenoid coil is excited, the armature is pulled in the stroke direction and, as a result of the first limit stop, takes the valve needle with it. When the current exciting the solenoid coil is switched off, the valve needle is accelerated into its closed position by the first resetting spring, taking the armature with it as a result of the above-mentioned limit stop. As soon as the valve-closure member strikes against the valve seat, the closing motion of the valve needle is abruptly ended. The armature, which is not fixedly joined to the valve needle, continues to move in the stroke direction and the motion is absorbed by the second resetting spring, i.e., the armature swings through against a second resetting spring, which has a significantly lower spring constant than the first resetting spring. Finally, the second resetting spring accelerates the armature once again in the stroke direction. If the armature strikes against the limit stop of the valve needle, the can result in the valve-closure member, which is connected to the valve needle, once again briefly lifting off from the valve seat, therefore generating a short-term opening of the fuel injector. The debouncing in the fuel injector known from German Published Patent Application No. 33 14 899 is therefore imperfect. In addition, it is disadvantageous both in a conventional fuel injector, in which the armature is fixedly connected to the valve needle, as well as in the fuel injector known from German Published Patent Application No. 33 14 899, that the opening stroke of the valve needle begins immediately as soon as the magnetic force exerted by the solenoid coil on the armature exceeds the sum of the forces acting in the closing direction, i.e., the spring closing force exerted by the first resetting spring and the hydraulic forces of the fuel, which is under pressure. This is disadvantageous to the extent that, when the current exciting the solenoid coil is switched on, the magnetic force has not yet reached its final value due to the self-induction of the solenoid coil and the eddy currents that arise. At the beginning of the opening stroke, the valve needle and the valve-closure member are therefore accelerated at a reduced force. This leads to an opening time that is not satisfactory for all application cases.
In U.S. Pat. No. 5,299,776, it is proposed, in this regard, not to connect the armature fixedly to the valve needle but rather to allow the armature a certain axial freedom of motion on the valve needle. However, in this embodiment, the axial position of the armature, in the resting position of the fuel injector, is not defined, and therefore, in the fuel injector known from this publication, the response time is uncertain when the exciting current is switched on.
In contrast, the fuel injector according to the present invention has the advantage that the fuel injector is debounced in a satisfactory manner and, additionally, that it has an extremely small opening time. Because of the immediate limit stop on the connecting part, an adjusting or guide disk can be dispensed with. The enlarged guide diameter generates improved guide properties, i.e., the valve needle is less sensitive to tilting or jamming. As a result of the fact that the guide is closer on the armature, the moments are reduced.
As a result of the fact that the second resetting spring holds the armature in the resting position of the fuel injector not at the first limit stop provided on the valve needle but at a stationary second limit stop at a distance from the first limit stop of the valve needle, it is achieved, when the fuel injector is closed, that the armature is not accelerated once again in the stroke direction by the second resetting spring. When the fuel injector is closed, the motion of the valve needle is abruptly ended if the valve-closure member comes into contact with the valve seat, as a result of the acceleration by the first resetting spring. In the fuel injector according to the present invention, the motion of the armature also continues in the closing direction, i.e., contrary to the stroke direction, until the armature has reached the second limit stop. If the armature rebounds from the second limit stop, it is nevertheless once again accelerated by the second resetting spring, contrary to the stroke direction, and the armature is prevented once again from reaching the first limit stop on the valve needle and therefore from carrying the valve needle with it in the opening direction. As a result of the second resetting spring, the armature is held at a distance from the first limit stop provided on the valve needle until the armature is once again accelerated in the stroke direction by the solenoid coil as a consequence of the next current pulse exciting the solenoid coil.
A further advantage of the fuel injector lies in the fact that the armature initially experiences a preacceleration before reaching the first limit stop provided on the valve needle, i.e., before carrying the valve needle with it. In this manner, the armature received an impulse, which it transmits to the valve needle, even before carrying the valve needle with it. In comparison to a fuel injector in which the armature is fixedly connected to the valve needle or a fuel injector in which the armature is movable with respect to the valve needle but in the resting position contacts the limit stop of the valve needle, a substantially shorter opening time and therefore a more precise metering of the fuel is achieved. A further effect that shortens the opening time arises from the fact that the magnetic force exerted on the armature is initially reduced when the current pulse exciting the solenoid coil is switched on, due to the self-induction of the solenoid coil and the eddy currents excited by the solenoid coil. If the distance is appropriately dimensioned between the second limit stop, at which the armature is positioned in its resting position, and the first limit stop, which acts to carry the valve needle with it, it can therefore be achieved that, when the armature strikes the first limit stop of the valve needle, so much time has already passed that the magnetic force has reached its final constant value. Therefore, as a result of the added flight time of the armature, a time delay is achieved which significantly shortens the subsequent opening time of the fuel injector.
In the connecting part executed as the valve seat support, it is possible to integrate both an armature guide as well as a second limit stop. In this context, however, in a first peripheral area, the valve seat support having the armature guide can be configured not having the second limit stop, and in a second peripheral area, it can be configured as having the second limit stop but not having the anchor guide. As a result of the functional separation of limit stop and anchor guide, the requirements with respect to manufacturing precision are less stringent.
To prevent a magnetic short-circuit, the magnetic restricter is preferably arranged in the area between the second limit stop and a valve needle shaft of the valve needle. The magnetic restricter can be arranged either on the valve seat support or on the armature. In the arrangement on the armature, the magnetic restricter is preferably located on the periphery of a segment aligned with the valve needle.